GABA-modulin (GM), a basic protein operative in mediating BABA benzodiazepine receptor interaction, was present in high concentrations in association with GABA recognition sites in rat brain synaptosomes. GM is basic in nature and is rich in arginine and lysine residues. The above characteristics link this peptide to the general class of brain basic peptides. However physiochemical and biochemical observations indicate that GM is different from any other known basic peptide. This difference has been confirmed by using a monoclonal antibody which reacts with GM, but not with any other basic brain protein. With this antibody, it is possible to demonstrate the presence of high concentrations of GM in neurons rich in GABA receptors.